Non-aqueous electrolyte secondary batteries used as electric power sources for portable electronic equipments in recent years include a lithium-containing transition metal oxide in the positive electrode and a carbon material capable of absorbing and desorbing lithium in the negative electrode, thereby having high power and high energy density. The positive and negative electrodes include respective binders for binding active material particles together. As the binder in the negative electrode, used are polyvinylidene difluoride (PVDF) or styrene-butadiene rubber (SBR), for example.
To impart sufficient strength to the negative electrode, it is necessary to mix a large amount of binder with a negative electrode active material such as the above carbon material. If a large amount of binder is used, however, the surface of the carbon material is covered with the binder. This reduces the surface area of the carbon material contributable to charging and discharging reaction, and thus deteriorates the high-rate discharge characteristics and the low-temperature characteristics of the battery. To compensate this, the salt concentration in the non-aqueous electrolyte must be increased.
However, increase of the salt concentration will enhance the reactivity of the electrolyte when the battery is under a high temperature and overcharged. Therefore, the battery temperature tends to easily rise, and thus the safety may be impaired.
In addition, if most of the surface of the carbon material is so covered with the binder that the surface area of the carbon material contributable to the charging and discharging reaction reduces, the carbon material fails to absorb a sufficient amount of Li. As a result, metallic Li is deposited on the surface of the carbon material, and thus the safety of the battery may further be impaired.
Also, the high-rate discharge characteristics of the batteries are greatly influenced by the affinities between the non-aqueous electrolyte and the electrodes, which depend on the amount and kind of the binders. If the permeability of the non-aqueous electrolyte into one of the electrodes is too high, the distribution of the non-aqueous electrolyte inside the battery is nonuniform, and thus the high-rate discharge characteristics are impaired.